In the preceding code, Istanbul instruments our source code in the lib
folder, so anything outside of lib won’t be taken into account by the report.
The original folder is not modified, just renamed temporarily while the report
is generated. Next, lib is restored to its original name and we get rid of
other folders generated in the process.

You should change lib to the location of your source code.

As a nice final touch, we can add two extra actions to our Makefile, clean
and test:

clean:
rm -rf reports
test:
$(MOCHA) -R spec $(TESTS)

With this, we now have available a coverage action in the Makefile, and we can
execute the following in the root of our project folder:

make coverage

and Istanbul will generate a complete report in the report folder. You can
now go there and open index.html to see your coverage by lines, functions and
files.